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Oversized pistons will indicate the overbore with the amount of overbore, as in .020 lightly stamped into the top of the piston.

The reason for the compression test with the engine warmed up is to allow the cylinders to be lubricated and any imperfections which could alter the sealing with a cold engine over a warm engine because it doesn't matter what the compression is when the engine is cold, it is what it is when the engine is warm. A really well assembled engine may not have much difference, but a worn engine could have greater than the magical ten percent difference between cylinders, so remember, we don't drive around with cold engines anyway, so, if greater than ten percent cold, warm the engine up and check again, and if it is the same, then you can know for sure there is a compression or sealing problem.
 

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Did you mic or at least caliper the cylinders to determine if bore is greater than 4.00 or 4.020? That much difference does sound like rings. Any scratches in the cylinder walls? Sometimes rings that are broken don't show scratches.
 

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Not unless it actually damaged the ring, and to do that it would have had to pit and break, and an oil check would show a difference. If it were that bad it wouldn't have been cleaned up with a light hone.
 

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Won't know until you pop the piston on the two cylinders out. If the head gasket had gotten a leak between the two cylinders and water entered the hot cylinders, the rings could have instantly laxed and the pressure drop just like that, and you would have no way of knowing for sure without the history of the engine. This wouldn't require any machining at all, but the rings on the pistons when the pistons were pulled would be tighter in the lan grooves and not expand, or expand softly when you pull them out of the cylinders but otherwise look fine.
 
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